Methods, systems, and apparatus, including an apparatus for using probabilistic techniques to provide reformatted versions of digital components. In one aspect, a process includes obtaining data specifying a distribution parameter limit for a given reformattable digital component that is eligible for reformatting using a set of digital component extensions. For each of multiple digital component requests, a determination is made that a given digital component extension has an additional selection requirement that, when combined with a base selection requirement for the given reformattable digital component, would exceed the distribution parameter limit. A determination is made, using a probabilistic technique, a probability at which the given digital component extension will be selected for use in generating a reformatted version of the given reformattable digital component such that an aggregate selection requirement for distributing the given reformattable digital component in response to requests over time is within the distribution parameter limit.